This invention relates to hydraulic systems for controlling the output of at least one variable displacement pump driven by a prime mover in accordance with speed and load conditions.
Many hydraulically actuated work performing systems, as, for example, hydraulic excavators, typically employ a plurality of variable displacement pumps for supplying high pressure hydraulic fluid to a plurality of motors utilized for performing a variety of functions. Typically, the prime mover for driving the pumps, such as an internal combustion engine, is normally set to operate at a predetermined speed when the system is in normal operation. Not infrequently, it is desirable to operate the vehicle at a different speed than the predetermined speed and, in many systems, as soon as the engine speed is reduced, the underspeed valve employed therein signals the pumps to reduce the displacement thereof since the underspeed valve responds to the change in speed as though the engine output was slowed due to lugging.
As a consequence, at speeds reduced from normal, work performing functions may be unnecessarily slowed even though there is ample horsepower in the system for handling the then prevailing load.
In order to overcome this deficiency, it has been proposed to provide a selectively operable, adjustable bypass for the underspeed valve to accommodate lower than normal speed operations without reducing the displacement of the variable displacement pumps. While such a system works well for its intended purpose, it requires the presence of additional components as, for example, conduits, valves, adjustable orifices, etc., and accordingly increases the cost of the overall hydraulic system.